Get an airsoft grenade and roll it into the room when your parents are having Xmas dinner and yell 'fire in the hole!' When the BBs hit them, point out that your were able to get a two kills without pointing a gun at them and no guns were pointed at you either. Sorted.
A bit of sideways thinking, but maybe the best way to convince them that it's so far away from the real thing is to actually take them for a game? Let them see for themselves? Could be both hilarious and educational...
